This invention relates to ultrasound devices and methods for imaging internal portions of the human body, and more particularly, to interventional intravascular or intracardiac imaging using catheters with multi-element transducer arrays.
Ultrasound imaging has been widely used to observe tissue structures within a human body, such as the heart structures, the abdominal organs, the fetus, and the vascular system. Ultrasound imaging systems include a transducer array connected to multiple channel transmit and receive beamformers applying electrical pulses to the individual transducers in a predetermined timing sequence to generate transmit beams that propagate in predetermined directions from the array. As the transmit beams pass through the body, portions of the acoustic energy are reflected back to the transducer array from tissue structures having different acoustic characteristics. The receive transducers (which may be the transmit transducers operating in the receive mode) convert the reflected pressure pulses into corresponding RF signals that are provided to the receive beamformer. Due to different distances to the individual transducers, the reflected sound waves arrive at the individual transducers at different times, and thus the RF signals have different phases. The receive beamformer has a plurality of processing channels with compensating delay elements connected to a summer. The receive beamformer uses a delay value for each channel and collect echoes reflected from a selected focal point. Consequently, when delayed signals are summed, a strong signal is produced from signals corresponding to this point, but signals arriving from different points, corresponding to different times, have random phase relationships and thus destructively interfere. Furthermore, the beamformer selects the relative delays that control the orientation of the receive beam with respect to the transducer array. Thus, the receive beamformer can dynamically steer the receive beams that have desired orientations and focus them at desired depths. In this way, the ultrasound system acquires echo data.
Invasive, semi-invasive and non-invasive ultrasound systems have been used to image biological tissue of the heart and the vascular system. Doppler ultrasound imaging systems have been used to determine the blood pressure and the blood flow within the heart and the vascular system. The semi-invasive systems include transesophageal imaging systems, and the invasive systems include intravascular imaging systems. A transesophageal system has an insertion tube with an elongated semi-flexible body made for insertion into the esophagus. The insertion tube is about 110 cm long, has about a 30 F diameter and includes an ultrasonic transducer array mounted proximate to the distal end of the tube. The transeophageal system also includes control and imaging electronics including the transmit beamformer and the receive beamformer connected to the transducer array. To image the heart, the transmit beamformer focuses the emitted pulses at relatively large depths, and the receive beamformer detects echoes from structures located 10-20 cm away, which are relatively far in range.
The intravascular imaging systems use an intravascular catheter that requires different design considerations from a transeophageal catheter. The design considerations for an intravascular catheter are unique to the physiology of the vascular system or to the physiology of the heart. The intravascular catheter has an elongated flexible body about 100-130 cm long and about 8F to 14F in diameter. The distal region of the catheter includes an ultrasonic transducer mounted proximate of the distal end. To image the tissue, several mechanical scanning designs have been used. For example, a rotating transducer element or a rotating ultrasound mirror is used to reflect the ultrasound beam in a sweeping arrangement. Furthermore, catheters with several transducer elements have been used, wherein different transducer elements are electronically activated to sweep the acoustic beam in a circular pattern. This system can perform cross-sectional scanning of arteries by sweeping the acoustic beam repeatedly through a series of radial positions within the vessel. For each radial position, the system samples the scattered ultrasound echoes and stores the processed values. However, these ultrasound systems have a fixed focal length of the reflected acoustic beam. The fixed focal length significantly limits the resolution to a fixed radius around the catheter.
Furthermore, intravascular ultrasound imaging has been used for determination of the positions and characteristics of stenotic lesions in the arteries including the coronary arteries. In this procedure, a catheter with a transducer located on the tip is positioned within an artery at a region of interest. As the catheter is withdrawn, the system collects ultrasound data. The imaging system includes a catheter tracking detector for registering the position and the velocity of the transducer tip. The imaging system stacks two-dimensional images acquired for different positions during the transducer withdrawal. An image generator can provide three-dimensional images of the examined region of the blood vessel or the heart, but these images usually have low side penetration.
Recently, ultrasound catheters with the above-described mechanical, rotating transducer designs have increasingly been used in the assessment and therapy of coronary artery diseases. These catheters have a larger aperture, giving rise to deeper penetration depths, which allows imaging of tissue spaced several centimeter away from the transducer, such as the right atrium of the human heart. These images can assist in the placement of electrophysiology catheters. However, these devices still do not provide high quality, real time images of selected tissue regions since they have somewhat limited penetration, a limited lateral control and a limited ability to target a selected tissue region. In general, the produced views are predominantly short axis cross-sectional views with a low side penetration.
Currently, interventional cardiologists rely mainly on the use of fluoroscopic imaging techniques for guidance and placement of devices in the vasculature or the heart as performed in a cardiac catheterization laboratory (Cathlab) or an electrophysiology laboratory (Eplab). A fluoroscope uses X-rays on a real-time frame rate to give the physician a transmission view of the chest cavity, where the heart resides. A bi-plane fluoroscope, which has two transmitter-receiver pairs mounted at 90xc2x0 to each other, provides real time transmission images of the cardiac anatomy. These images assist the physician in positioning the catheters by providing him (or her) with a sense of the three-dimensional geometry in his (or her) mind that already understands the cardiac anatomy. While fluoroscopy is a useful technique, it does not provide high quality images with real tissue definition. The physician and the assisting medical staff need to cover themselves with a lead suit and need to limit the fluoroscopic imaging time when ever possible to reduce their exposure to X-rays. Furthermore, fluoroscopy may not be available for some patients, for example, pregnant women, due to the harmful effects of the X-rays. The transthoracic and transesophageal ultrasound imaging techniques have been very useful in the clinical and surgical environments, but have not been widely used in the Cathlab or Eplab for patients undergoing interventional techniques.
What is needed, therefore, is an ultrasound system and method for effective intravascular or intracardiac imaging that can visualize three-dimensional anatomy of a selected tissue region. Such system and method would need to use an imaging catheter that enables easy manipulation and positional control. Furthermore, the imaging system and method would need to provide convenient targeting of the selected tissue and good side penetration allowing imaging of near and more distant tissue structures, such as the right and left sides of the heart.
The present invention is an ultrasound system and method for intravascular imaging. According to one aspect, an ultrasound system for imaging biological tissue includes an intravascular catheter with an ultrasound transducer array, a transmit beamformer, a receive beamformer, and an image generator. The intravascular catheter has an elongated body made for insertion into a blood vessel and connected to a catheter handle. The catheter includes a catheter core located inside a steerable guide sheath, both having a proximal part and a distal part. The catheter includes an articulation region connected to a positioning device for positioning the transducer array to have a selected orientation relative to an examined tissue region. For each orientation of the transducer array, the transmit and receive beamformers acquire ultrasound data over an image plane of the examined tissue region. The catheter core is connected to a rotation device constructed and arranged to rotate, or oscillate over an angular range, the transducer array that acquires ultrasound data over a multiplicity of image planes. The image generator is constructed to form a selected tissue image based on the acquired ultrasound data.
According to another aspect, in an ultrasound system for imaging biological tissue, including an array of ultrasound transducers connected to transmit and receive beamformers constructed to obtain an ultrasound image of a selected tissue region, providing an intravascular catheter. The intravascular catheter includes an imaging core and a steerable guide sheath. The steerable guide sheath includes a distal sheath part and a proximal sheath part constructed for insertion into a blood vessel. The distal sheath part includes an articulation region constructed to assume a selected orientation. The imaging core includes a distal core part, located within the distal sheath part, and a proximal core part, located within the proximal sheath part, and being constructed for rotational motion inside the guide sheath. The ultrasound transducer array is disposed longitudinally on the distal core part of the imaging core. A positioning device is constructed to control the selected orientation of the articulation region and thereby orient the ultrasound transducer array relative to the selected tissue region. The ultrasound transducer array is constructed to detect ultrasound data over an image sector defined by an azimuthal angular range. A rotation device constructed to rotationally displace, over an elevation angular range, the ultrasound transducer array about the apex of the image sector.
According to another aspect, an ultrasound system for imaging biological tissue includes a catheter with a catheter handle, a transmit beamformer, a receive beamformer, and an image generator. The catheter includes core means including an ultrasound transducer array disposed longitudinally on a distal part of the core means, guide sheath means for receiving the core means and enabling defined rotational movement of the core means. The catheter also includes articulation means, connected to positioning means, for orienting the transducer array relative to a tissue region of interest and rotation means, connected to the core means, for oscillating the transducer array over a selected elevation angular range. The transmit beamformer and the receive beamformer are connected to the transducer array and constructed to acquire, for each elevation angle of the transducer array, ultrasound data of an image sector defined by an azimuthal angular range. The image generator is constructed to receive ultrasound data over a multiplicity of image sectors for different elevation angles and to form an image of the tissue region of interest from the ultrasound data.
Preferred embodiments of these aspects include at least one of the following features:
The ultrasound system and the catheter are constructed to collect the ultrasound data over a selected volume defined by an azimuthal angular range and an elevation angular range. The ultrasound system and the catheter are constructed and arranged for real-time imaging capable of achieving a scanning frequency of at least 15 Hz.
The imaging core and the steerable guide sheath are connected to a catheter handle. The catheter handle further includes a rotation device and a compensation mechanism. The compensation mechanism is arranged to counter balance the motion of the rotation device in order to reduce unwanted vibrations in the handle. The accelerometer may provide a signal to the compensation mechanism.
The rotation device includes a drive motor connected to the imaging core and the compensation mechanism includes a counter balance motor. The compensation mechanism is designed to have a natural frequency response at a frequency of oscillation of the ultrasound array.
The rotation device includes a drive motor constructed and arranged to oscillate the imaging core at varying frequencies above a resonance frequency of about 15 Hz.
The rotation device includes a drive motor constructed and arranged to oscillate the ultrasound array over selected angles of the elevation angular range. The rotation device is further constructed and arranged to position the ultrasound array at a selected angle relative to the tissue region of interest and maintain the array at the angle for a selected period of time. The rotation device may include a stepper motor connected to the imaging core.
The intravascular catheter further includes a set of bearings disposed between the imaging core and the guide sheath and arranged to facilitate the rotation or oscillation of the ultrasound array about the apex of the image sector. The bearings may have a low profile and may be molded into the guide sheath. The bearings may have a hydrostatic design.
The catheter handle includes an accelerometer connected to the compensation mechanism and arranged to detect unwanted vibrations in the handle. The accelerometer may provide a signal to the compensation mechanism.
The catheter includes a position sensor constructed and arranged to detect orientation of the ultrasound array and provide a feedback signal to the rotation device. The position sensor may be located in the distal sheath. The position sensor may include an acoustic time-of-flight positioning system with a transmitter and a detector. The position sensor may include an AC electromagnetic tracking sensor or a DC electromagnetic tracking sensor.
The catheter includes an accelerometer sensor arranged to detect vibrations due to the movement of the imaging core.
The articulation region of the catheter includes a multiplicity of articulation links cooperatively arranged with a first articulation mechanism. The first articulation mechanism includes at least one push-pull rod connected to the positioning device. The catheter may further include a sensor constructed and arranged to detect displacement of the push-pull rod. The positioning device may include a rack and pinion mechanism. The articulation links and the push-pull rod are cooperatively arranged to flex in-plane the distal portion upon actuation by the positioning device. The articulation region may form an in-plane J hook.
The catheter further includes a second articulation mechanism. The second articulation mechanism includes a second push-pull rod cooperatively arranged with the articulation links to flex out-of-plane the distal portion (i.e., an out-of-plane J hook) upon actuation by the positioning device.
The catheter includes two push-pull rods and a multiplicity of articulation links included in the articulation region. The multiplicity of links are cooperatively arranged with the push-pull rods to flex in-plane the distal portion to form an S-like curve upon actuation of the push-pull rods by the positioning device. The catheter may further include a third articulation mechanism. The third articulation mechanism includes a third push-pull rod cooperatively arranged with the articulation links to further flex out-of-plane the distal portion (i.e., an out-of-plane S hook) upon actuation by the positioning device.
The imaging core includes a drive shaft constructed to exhibit a high torsional stiffness and a high bending flexibility. The drive shaft may be made of at least two counter wound springs.
The imaging core is removably insertable into the steerable guide sheath. The steerable guide sheath is connectable to a sheath handle, which is connectable to the catheter handle. The sheath handle may further include a v-band clamp constructed and arranged to lock into position the guide sheath relative to the handle. The guide sheath may be disposable or reusable upon cleaning and sterilization. The guide sheath further includes an ultrasonically transparent window located in front of the transducer array.
The catheter further includes a filling port constructed and arranged to provide a coupling medium between the distal sheath part and the distal core part. The filling port may be located near the catheter handle.
The catheter further includes a flush port located in the distal sheath part and arranged in communication with the volume between the distal sheath part and the distal core part.
The position sensor may be constructed and arranged to provide a feedback signal providing the position of the imaging core to the drive motor. The drive motor includes a rotary encoder constructed and arranged to provide an angular position feedback.
The ultrasound system may be constructed and arranged to perform a four dimensional scan of the tissue volume.
According to another aspect, a method for imaging biological tissue includes inserting into a blood vessel an elongated body of an intravascular catheter with an ultrasound transducer array positioned longitudinally on a distal part of the elongated body. The transducer array is connected to a transmit beamformer and a receive beamformer. The method also includes positioning the transducer array to have a selected orientation relative to an examined tissue region, and for each orientation of the transducer array, acquiring ultrasound data over an image plane of the examined tissue region. The method also includes rotating, or oscillating over an angular range, the transducer array and acquiring ultrasound data over a multiplicity of the image planes, and forming a selected tissue image of the tissue region based on the acquired ultrasound data.
Advantageously, the articulation mechanism, located preferably within the catheter sheath, orients the transducer array located preferably on the distal part of the imaging core. The ultrasound system collects echo data over a selectable predictable tissue volume and provides a corresponding data volume. The selectable location and orientation of the data volume improves significantly the tissue images. A clinician may select rotation speed or scanning frequency of the ultrasound array to collect two-dimensional images of selected tissue including a moving organ. The imaging system enables understandable visualization of the tissue providing images with a known orientation. A video display provides anatomically correct orientation of the images.
Furthermore, there are several advantages to positioning the transducer array near the tissue of interest and performing near-in field imaging (as opposed to far-in field imaging performed by the non-invasive or semi-invasive ultrasound system, i.e., transthoracic or transesophageal ultrasound systems). For example, placing the transducer close to the tissue of interest substantially reduces the number of scattering, absorbing, and aberating tissue structures, which degrade acoustic images.
The intravascular catheter has a small diameter that fits through the vascular system used to position the transducer array relatively close to the tissue of interest, for example, the heart tissue. This small diameter dictates a small elevation aperture of the transducer array. The small aperture requires a higher ultrasound frequency to reduce the beam width and thus improve resolution. While higher frequencies are absorbed more rapidly in the tissue, here this is not a problem because the transducer array is positioned relatively close to the tissue of interest. The beam width also varies with the range, but focussing of the beam improves the beam width at the point of focus. A low f-number is required for improved resolution. For an adequate depth of field, acoustic images generally require an f-number of about 2 to 4, which places the area of best resolution (or focus) at two to four times the aperture; this corresponds to the range of 4 mm to 8 mm in the elevation direction.
There are additional benefits of the intravascular imaging. The transducer is always surrounded by blood thereby enabling perfect acoustic coupling to surrounding myocardium or other vessels, organs or tissue being imaged. On the other hand, a poor acoustic contact in transesophageal and transthoracic imaging can create problems ranging from intermittent loss of image to complete inability to acquire an image. This can increase diagnostic time and be devastating to an interventionalist relying on real time echo data to guide intervention devices.